WHAT: In a new study of nearly one million adults between the ages of 18 and 64, nearly 70 percent of participants underwent at least one medical imaging procedure between July 2005 and December 2007, resulting in an average effective dose of radiation nearly double the amount they would otherwise be exposed to from natural sources. Nearly 20 percent of participants received at least moderate annual doses of radiation from diagnostic tests, and women and older individuals were at greater risk for radiation exposure, according to a report in the August 27 issue of the New England Journal of Medicine.

Retroviruses such as HIV that are already within cells are much more easily transmitted when they are next to uninfected cells than if they are floating free in the bloodstream.

"Cell-to-cell transmission is a thousand times more efficient, which is why diseases such as AIDS are so successful and so deadly," said Walther Mothes, associate professor of microbial pathogenesis at the Yale School of Medicine. "And because the retroviruses are already in cells, they are out of reach of the immune system."

Fallout from nuclear bomb tests during the cold war has just yielded encouraging news for those searching for ways to reverse heart disease.

A team led by Jonas Frisén from the Karolinska Institute in Stockholm has shown that adult human hearts make new muscle cells, albeit very, very slowly1. Human heart cells that can generate cardiomyocytes in culture have been identified before. But how the heart regenerates naturally has been hotly contested, says Kenneth Chien of the Harvard Stem Cell Institute in Cambridge. "This study shows for the first time and very clearly that there is some turnover of cardiomyocytes within the lifetime of an individual." It also lays to rest claims that heart cells turn over quickly, says Deepak Srivastava of the Gladstone Institute of Cardiovascular Disease in San Francisco, California.

Every day, haematopoietic stem cells replenish the blood system, but they only make as much blood as the body needs. Although much is known about how haematopoietic stem cells move through both self-renewal and differentiation to become many blood types, far less is understood about what keeps the cells in a nondividing state. Now, researchers led by Stephen Nimer at the Memorial Sloan-Kettering Cancer Center in New York have elucidated a key role for p53 in the regulation of haematopoietic stem cell quiescence.

The blood-forming system is simultaneously the hardest and easiest to study. On the one hand, collecting cells can be as simple as a blood draw; on the other hand, haematopoietic stem cells do their real work — generating new cells — while they are tucked away in the bone marrow. Two studies published this month in Nature show how to get a sustained peek inside this niche, revealing the complex support system that guides haematopoietic stem cells to produce specialized progeny.